1. Field of Invention
The invention relates to a method for refreshing the injection law of a fuel injector [i.e., for refreshing the law that binds the actuation time (i.e., the driving time) to the injected-fuel quantity].
2. Description of Related Art
Patent Application EP2455605A1 suggests a method for determining the actual injection law of a fuel injector to be tested. The method includes the steps of: interrupting the feeding of fuel from the fuel pump to a common rail; avoiding the opening of all fuel injectors, except for the fuel injector to be tested; measuring the initial fuel pressure inside the common rail before starting the opening of the fuel injector to be tested; opening the fuel injector to be tested for a number of consecutive openings greater than one with a same test actuation time; measuring the final fuel pressure inside the common rail after ending the opening of the fuel injector to be tested; and estimating as a function of a pressure drop in the common rail the fuel quantity that is actually injected by the fuel injector to be tested when it is opened for the test actuation time.
Patent Application EP0488362A1 and Patent Application US2006107936A1 suggest methods for refreshing the actual injection law of a fuel injector to be tested.
As described in Patent Application EP2455605A1, during the normal operation of the internal-combustion engine, an electronic-control unit determines the required fuel quantity for each fuel injector as a function of the objectives of the engine-control unit and, thus, determines the desired actuation time for each fuel injector as a function of the desired fuel quantity by using the injection law stored in the electronic-control unit itself. In normal conditions, each fuel injector would be actuated using exactly the desired actuation time. Instead, for estimating, the electronic-control unit compares each test actuation time with the desired actuation time to establish whether at least one test actuation time is compatible with the desired actuation time and, thus, estimates the fuel quantity that is actually injected by the fuel injector when it is opened for a test actuation time if such a test actuation time is compatible with the desired actuation time.
A test actuation time is compatible with the desired actuation time if the fuel quantity injected with test actuation time is equal to a whole sub-multiple of the desired fuel quantity injected with the desired actuation time minus a “tolerance” interval [i.e., if the fuel quantity injected in the test actuation time multiplied by a whole number (including number 1) (i.e., the test actuation time may be identical to the desired actuation time) is equal to the desired fuel quantity injected in the desired actuation time minus a “tolerance” interval (it is evidently very difficult to obtain perfect equality without allowing a minor difference)].
After having identified a test actuation time (minus the “tolerance” interval) compatible with the desired actuation time, the electronic-control unit modifies the desired fuel quantity required by the electronic-control unit in the “tolerance” interval so that the average fuel quantity corresponding to the test actuation time is exactly a sub-multiple of the desired fuel quantity (obviously, the average fuel quantity corresponding to the test actuation time could be identical to the desired fuel quantity). In other words, to estimate the fuel quantity injected by a fuel injector to be tested using a test actuation time, starting from the desired fuel quantity required by the engine control of the internal-combustion engine, the electronic-control unit may decide to modify (“override”) the injection features by varying both the desired fuel quantity (within the “tolerance” interval) and by dividing the injection into several consecutive injections.
However, it has been observed that replacing a single “long” injection (having a duration equal to the desired actuation time), which occurs in a linear operating zone of the fuel, with many consecutive “short” injections (each of which feeds a fuel quantity equal to a sub-multiple of the desired fuel quantity), which occurs in a ballistic operating zone of the fuel injector, may lead to a significant total error of the fuel quantity that is actually injected (i.e., the fuel quantity that is actually injected by the series of “short” injections can be significantly different from the desired fuel quantity) because the injection errors of all the consecutive “short” injections are algebraically summed.
In other words, the error between the normal injection law and the actual injection law is always low when the fuel injector is used in the linear operating zone whereas the error between the nominal injection law and the actual injection law may be even very high when the fuel injector is used in the ballistic operating zone. Above all, at the beginning of the actual injection law of each fuel injector, the actual behavior of the fuel injector in the ballistic operating zone is not known with adequate accuracy. Thus, replacing single operation in the linear operating zone with multiple operation in the ballistic operating zone may imply very high errors in the injected-fuel quantity with major repercussions on the operating smoothness of the internal-combustion engine.
It is an object of the invention to provide a method for refreshing the injection law of a fuel injector, which method is free from the above-described drawbacks and, in particular, easy and cost-effective to implement and allows avoidance in any situation operating irregularities of the internal-combustion engine.